This invention relates generally to systems for the reproduction of recorded information, such as video information recorded on a disc, and, more particularly, to systems for maintaining a light beam essentially centered on an information track on such a disc.
Video and other types of information can be stored on a disc as a succession of light reflective and non-reflective regions along a spiral information track on the surface of a disc-shaped record carrier. In reproducing the video information, a video disc player employs an optical system for directing a radiant reading beam, such as a laser beam, onto the information track, and for detecting a reflected beam indicative of the reflectivity of the disc surface scanned by the laser beam as the disc is rotated. In a disc player of the type in which the invention may be usefully employed, the intensity of the reflected beam takes the form of a high-frequency carrier signal which is frequency modulated by the video information.
In order to store a reasonably large amount of information on the disc, successive turns of the information track must be so closely spaced that even a slight degree of disc eccentricity would cause the beam to transverse a number of adjacent tracks on each revolution. Consequently, some means must be provided for applying transverse or radial corrections to the beam position, so that it substantially follows along the center of the information track.
Applying conventional analog servo system theory to this problem presents some difficulty, since the response characteristic of the disc is symmetrical about the track centerline, and the system must determine in which direction to apply a correction when the beam deviates from the centerline. An alternative but generally unsatisfactory analog approach is to select a non-central position as a null point for the servo system. Other disadvantages of analog servo systems are their relative complexity, and the necessity for precise and continual adjustment of components. Consequently, there is still a real need for a simple and reliable technique for maintaining a reading beam substantially in the center of an information track. The present invention has as its principal objective a digital system, as opposed to an analog system, for controlling the position of the beam with respect to the track centerline.